1. Field of the Invention
The subject invention relates to cardiac pacemaker systems and, more particularly, implantable cardiac pacemakers which deliver pacing stimulus pulses at an adjustable rate based upon monitoring of patient conditions.
2. Description of the Background and Prior Art
Rate responsive pacemaker systems are widely available in the art. Rate responsive systems contain means for monitoring at least one patient variable and for determining an indicated pacing rate as a function of such sensed pacing variable, so as to control pacing rate optimally in terms of the patient condition. Such rate responsive pacemakers have gained wide acceptance as providing an improved response to the patient's physiological needs, as compared to programmable fixed rate pacemakers. Although atrial-based pacemakers, i.e. atrial synchronous or atrial sequential pacemakers, as well as DDD pacemakers, may in some patients provide an ideal form of rate responsiveness, such pacemakers are not satisfactory for many patients with cardiac conditions.
A number of patient variables or rate control parameters have been suggested in the technical literature and used commercially. One of the first physiological parameters utilized for rate control is the QT interval, as disclosed in the U.S. Pat. No. 4,228,803 to Rickards, and the U.S. Pat. No. 4,305,396 to Wittkampf et al. The QT interval is in fact the interval between a delivered pacing stimulus and the subsequent evoked T-wave, and has been utilized as the parameter indicative of physiological demand for heart output, and thus pacing rate. Additionally, activity sensors have been widely utilized for detecting the general activity level of a patient with a pacemaker, and for controlling the pacing rate or escape interval in response to detected activity level. See the U.S. Pat. No. 4,428,378 to Anderson et al. Other parameters which have been utilized or investigated for suitability as controlling pacing rate include respiration rate, thoracic impedance changes, venous blood temperature, pH, oxygen saturation and stroke volume.
In addition to the selection of a desired monitored parameter, and the corresponding sensor to be used, the algorithm utilized by a pacing system is of great importance. An example of an improved rate adaptive algorithm used in a pacing system is set forth in U.S. Pat. No. 4,972,834, Ser. No. 252,653, filed Sept. 30, which discloses a QT pacemaker with dynamic rate responsiveness, incorporated herein by reference. As set forth in this referenced patent, the algorithm which correlates the monitored or sensed parameter with indicated pacing rate may be adapted as a function of history, and particularly can be readjusted with respect to limits such as lower rate limit (LRL) and upper rate limit (URL).
Another approach to optimizing rate responsiveness is to use dual or plural sensors, in order that the drawbacks or deficiencies of a given sensor and/or algorithm may be compensated by the use of a second or other sensors having different characteristics. This approach is set forth in the patent to Rickards, U.S. Pat. No. 4,527,568, which discloses switching control of rate responsiveness from one monitored parameter, e.g. atrial rate, to another control parameter, e.g. QT interval. There are many other examples of dual sensor approaches in the literature, and reference is made to U.S. Pat. Nos. 4,926,863 and 4,905,697. These references are characterized by designs which switch control from one sensor to another, or from one algorithm to another, depending upon monitored values of the rate control parameters. While this approach may produce increased efficiency and improvement over the single sensor approach, it still does not provide a continuous optimization of information such as is potentially available from two or more sensors, so as to continuously optimize and adapt the actual pacing rate for all foreseeable conditions. As used in this specification, "sensor" or "sensor means" refers to any means for obtaining a control parameter, including the lead means such as is used for obtaining the QT interval, or other sensors such as in use for detecting body activity and the like. The techniques for sensing rate control parameters, and developing and processing therefrom signals useful for pacemaker control, are well known in the art.
A longstanding unsolved problem in this art area, for which there is a need for improvement, is thus to provide either a sensor or combination of sensors which more nearly fulfills the requirement of the ideal rate adaptive system. For example, a rate adaptive system should provide a quick and accurate initial response to situations such as start of exercise. The QT interval, as a rate control parameter, provides only a gradual response, as compared to an activity sensor which provides a fast, i.e., quick response. Another requirement is that the parameter or parameters chosen should provide an indication of pacing rate which is proportional to the work load. The QT interval provides a very good indication of work load, whereas the activity sensor approach is not as good, and may be subject to false indications. Another important requirement for a rate control parameter is specificity, i.e., that the characteristics of the parameter signal are specific to the conditions of rest and exercise of the patient and are thus physiologically appropriate. For example, the QT interval has a high specificity, whereas activity as a parameter has a medium specificity. Yet another requirement is providing an optimum indication of rate decrease following cessation or reduction of the condition compelling higher rate, such as exercise. It is important that the speed of rate decay after the cessation of exercise be properly related to the patient's physical condition. It is known that a patient in relatively poor physical condition experiences a slow decrease of the heart rate after exercise, while a person in relatively good physical condition experiences a more rapid decrease of the heart rate after exercise. A pacemaker controlled by an activity sensor is less than optimal in this regard, since a cessation of exercise results in a sharp drop in the activity signal which, if not modified, would lead to a non-physiological step-like reduction in pacing rate. As a consequence, it is necessary to program a fixed time period for gradually decreasing the pacing rate when the activity sensor stops delivering information calling for a higher rate. The pacemaker which is controlled by the QT interval exhibits the inverse relationship as known from exercise physiology, but tends to provide too slow a pacing rate decay.
What is thus sought in this art area is a pacer having two or more sensors and an algorithm for deriving information from each so as to optimize the determination of desired pacing rate. At the start of exercise, for example, it is desired to have the algorithm force an initial but limited fast rate increase. Thereafter, it becomes important to ensure that the pacing rate correlates proportionally to work load, and that if continuous exercise is not confirmed, the pacing rate will slowly decrease toward a lower limit. The algorithm, combined with the sensing means, should also force a faster, although limited rate decrease when stop of exercise is detected, with further rate decrease following the physiologically inverse relationship.
As is well known, the microprocessor and logic circuit technology for dealing with these problems in a pacemaker environment is available. What is needed is a pacemaker system which utilizes this technology so as to optimize the translation of plural sensor information into pacing rate control.